This operation constructs a lookup table to convert tensor of strings into
int64 IDs. The mapping can be initialized from a string mapping 1-D tensor
where each element is a key and corresponding index within the tensor is the
value.

Any lookup of an out-of-vocabulary token will return a bucket ID based on its
hash if num_oov_buckets is greater than zero. Otherwise it is assigned the
default_value.
The bucket ID range is [mapping size, mapping size + num_oov_buckets - 1].

The underlying table must be initialized by calling
session.run(tf.tables_initializer) or session.run(table.init) once.

Elements in mapping cannot have duplicates, otherwise when executing the
table initializer op, it will throw a FailedPreconditionError.